Prediction of Ductile-to-Brittle Transition Under Different Strain Rates in Undermatched Welded Joints

A welded joint generally has heterogeneity of strength, material, and fracture toughness. It is important to understand the characteristics of the strength and fracture of welded joints while considering the heterogeneous effect. In particular, the material behavior becomes more complicated when the welded joint with strength heterogeneity is subjected to dynamic rapid loading; for example, the welded heat-affected zone of pipeline steels is softened and welded underground pipelines are affected by a rapid ground sliding in an earthquake. In this paper, the characteristics of the strength and fracture of an undermatched joint under dynamic loading are studied by round-bar tension tests and thermal elastic-plastic analyses. The results show that the strength and fracture characteristics of the undermatched joints should be evaluated based on the effects of the strain rate and the temperature, including the temperature rise during dynamic loading. The tensile strength and the yield stress of the undermatched joints increase with the strain rate and with the decreasing temperature. The strength of the undermatched zone approaches that of the base metal when the thickness of the undermatched zone becomes smaller, and it does not depend on the strain rate. Finally, it is found that the stress-strain distribution affects fracture characteristics such as ductile-to-brittle transition behavior. The fracture characteristics are explained and predicted from the results of stress-strain relations obtained by numerical analysis.